Whether the reason is Pi’s are really difficult to get hold of, or you just needed more RAM on an existing Pi, it is technically quite possible to upgrade it yourself. The video in the linked article below demonstrates it being done.
You get a compatible RAM chip somewhere like Aliexpress – lucky for us, those come equipped with a set of balls from the factory. The default balls are made of lead-free solder, and MadEDoctor reballed the RAM chip with leaded solder balls to lower the melting point, but it’s by no means a requirement that you do the same.
In fact, you only need a hot air gun, flux, a soldering iron and some solder wick to perform the replacement – no reballing equipment.
The “brave” part is whether you feel comfortable heating the board up in this way and getting all those pins loosened, and the board cleaned. That said, I’ve heated my TV’s motherboard in the oven to solve a dry solder joint issue, and it is 100% many years later still.
See https://hackaday.com/2023/03/05/upgrade-ram-on-your-pi-4-the-fun-way/